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Posted: November 27, 2009

Polymers mop up isotopes to reduce radioactive waste

(Nanowerk News) Nuclear Power could solve our energy problems but it has rather nasty by-products: radioactive waste. And this does not only concern the old core rods whose disposal causes a lot of problems. Reactor operation also brings along extensive amounts of low-level waste, especially contaminated cooling water. This water also has to be costly disposed of in compliance with rigorous security restrictions. Together with his colleague Sevilimendu Narasimhan from the Bhabha Atomic Research Center in Kalpakkam, India, the chemist PD Dr. Börje Sellergren from the Institute of Environmental Research at Technische Universität Dortmund has developed a new method to reduce the amount of this radioactive waste considerably. His approach: small beads consisting of a special polymer which "fishes" the radioactivity out of the water.

In pressurized-water reactors, the most common reactor, hot water circulates at high pressure through the steel pipes, dissolving metal ions from the walls of the pipes. When the water is pumped through the reactor’s core, these ions are bombarded by neutrons.

Because the pipes are steel pipes, most of the ions are common iron-isotopes (56 Fe), which don’t become radioactive when bombarded by neutrons. But the steel in the pipes is usually alloyed with cobalt. And when this cobalt absorbs neutrons, an instable cobalt-isotope (60 Co) emerges which is radioactive with a half-life of more than five years.

Usually the water is cleaned with ion exchangers. But this technique has a crucial disadvantage, because it doesn’t differentiate between non-radioactive iron-ions and radioactive cobalt-ions.

To overcome this problem, Sellergren and Narasimhan were looking for a material which binds cobalt while ignoring iron. They developed a special polymer which is made through a procedure called “molecular imprinting”. This polymer is made in an environment containing cobalt. Then the cobalt-ions are extracted with hydrochloric acid, meaning that they are virtually “washed out”. The resulting cobalt-sized holes – the imprinting – are able to trap cobalt – and just cobalt – in other environments. The result: a small amount of this polymer can mop up a large amount of radioactive isotopes.

The team is now forming the polymer into small beads that can pass through the cooling system of a nuclear-power station. They expect that it would be more economical and environment-friendly to concentrate radioactivity into such beads than to dispose of large amounts of low-level waste. There obviously is a demand. Some 40 new nuclear-power stations are being built around the world. And the International Atomic Energy Agency estimates that a further 70 will be built in the next 15 years.